A coal strip mine in Utah: The U.S. leads the world in coal reserves, but not production. Getty Images

May 7, 2013

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First off: What exactly is coal, and how does it work?

Basically, coal is a combustible sedimentary rock that's formed when organic material decays under pressure and heat for millions of years. The end product — which forms layers of black or dark brown rock composed of carbon, hydrogen, sulfur, oxygen, and nitrogen, is the leading source of electricity generation in the world. In America, coal is responsible for more than 40 percent of our electricity.

When used as a solid fuel, coal is mined, transported, cleaned, and pulverized, then burned in a large furnace. Water heated by the burning coal converts into steam, which spins turbines, which turn generators, which, of course, produce electricity.

How bad is coal for the environment?

Pretty bad. Per unit of electricity, coal produces more pollution than any other fuel source, according to the U.S. Environmental Protection Agency. Not only does it pollute the air with carbon dioxide, mercury compounds, sulfur dioxide, and nitrogen oxides, but coal production also releases methane, a compound with global warming potential at least 23 times greater than carbon dioxide, the EPA says.

And to access coal, miners sometimes use a controversial process called "mountain top removal," which is about as detrimental to mountain ecosystems as it sounds. What's more, water used in the coal refining process collects heavy metals like lead and arsenic, which can contaminate groundwater and nearby surface water.

So-called "clean coal" does provide hope for environmentalists. For instance, according to Fox News, Ohio State's Clean Coal Research Laboratory discovered a way to release heat from coal without burning it, "instead using iron-oxide pellets for an oxygen source and containing the reaction in a small, heated chamber from which pollutants cannot escape." The process, "coal-direct chemical looping," still releases waste in the form of water and coal ash, but none in the form of greenhouse gasses. It would be expensive to implement, but such technologies are promising for advocates of greener energy sources.

Well, if coal is so bad for the environment, why do we use it?

For most big coal-producing countries, the upsides are largely economic and political. The U.S., for one, houses more than one-fourth of the world's coal reserves. There's more energy potential in American coal than in all the world's known recoverable oil reservoirs combined. Having this much coal means a lot of jobs— there are 87,520 coal workers in the U.S., according to the Bureau of Labor Statistics — and less dependence on foreign oil.

But how does the U.S. compare to the rest of the world when it comes to coal? Here's a breakdown of the seven biggest coal countries, based on reserves.